Nucleation of water vapor on condensation

Wallace and Hobbs, Sections 6.1 and 6.4

What besides water vapor do we need to make a cloud?

METR125: Physical Meteorology

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Courtesy: Steve Platnick, NASA

Homogeneous Nucleation of Droplets;Kelvin’s Equation

Cloud Condensation Nuclei. Warm Clouds.Growth of Drops by Condensation

Atmospheric Aerosols

Heterogeneous Nucleation of Droplets;Köhler Curves

Growth of Drops by Collisions. Ice Nuclei and Ice Crystal in Clouds

Growth of Ice Particles in Clouds

Rain Drops, Cloud Droplets, and CCN

Need to remember the typical size of CCN, cloud droplet, and raindrop

REVIEW

Review

• Clouds form when air becomes supersaturated wrt pure, flat liquid water (or ice, in some cases)

• Supersaturation most commonly occurs in the atmosphere when air parcels ascend, resulting in expansion and cooling (WH 2.6)

• Water vapor condenses onto aerosols forming a cloud of small water droplets

Aerosol

Theory• But do we really need (Andy) aerosol to

make a cloud droplet? Can we make cloud via condensation without the aid of aerosols*?

*homogeneous or spontaneous nucleation(last lecture)Very little, and take 24 hrs to grow to size of rain drop

Theory

• Kelvin’s formula can be used to– calculate the radius r of a droplet which will be

in (unstable) equilibrium with air with a given water vapor pressure e

– determine the saturation vapor pressure e over a droplet of specified radius r

• r = 0.01 micrometers requires a RH of 112.5%

• r = 1.0 micrometer requires a RH of 100.12%

Curvature Effect

• Supersaturations that develop in natural clouds due to the adiabatic ascent of air rarely exceed 1% (RH=101%),

• Consequently, droplets do not form in natural clouds by the homogeneous nucleation of pure water…

Theory

• …droplets do form in natural clouds by the heterogeneous nucleation process

• on atmospheric aerosolsYes!

Solution Effect • Once an aerosol particle

replaces a water molecule in the droplet, the equilibrium vapor pressure (e.g., number of water vapor molecules required to surround the droplet to maintain equilibrium) decreases

• Therefore, water droplet can keep from evaporate at lower vapor pressure

Aerosols

• PARTICLES, liquid or solid, SUSPENDED IN THE ATMOSPHERE

• DIAMETERS MEASURED IN MICRONS – ONE MILLIONETH OF A METER.– Too small to be seen individually (0.1 – 1 μm)

Size and shape of soot and sulfate aerosols in the atmosphere. Credit: Mihály Pósfai, Arizona State University

Sources of Aerosols : Wildfire in CA August 31, 2009

http://photojournal.jpl.nasa.gov/catalog/PIA12190

Sources of Aerosols: Volcanic Eruption

Mt. St. Helens, Washington, April 8, 1980

• The most important components of volcanic degassing are H2O, CO2 and sulfur as SO2, H2S and SO4- in varying fractions depending on the magma type

Dust storm in the Gobi Desert, China

Satellite: Terra Date : 3/9/2013Resolutions:

1km (691.1KB)500m (2.3 MB)250m (5.6 MB)

Bands Used: 1,4,3

http://modis.gsfc.nasa.gov/gallery/individual.php?db_date=2013-03-17

Skyscrapers in downtown Seoul in March 2006 are shrouded by yellow dust storms blowing in from China's Gobi desert.

Sources of Aerosols– Urbanizations

Aerosol effects on Clouds

• Video

http://www.met.sjsu.edu/metr112-videos/MET%20112%20Video%20Library-MP4/clouds/

The Clouds Makers.mp4

aerosol particles have a range of sizes from nanometers to micrometers, a range of shapes, and different chemical compositions

clouds cover roughly half of the earth's surface; yet any individual cloud is composed of billions of individual droplets or crystals, each of which started its life as an aerosol particle.

Size of Aerosol Particle

• Following the original work of Whitby (1978) and Hoppel (1988), it has been accepted to name these modes "nucleation mode" (particles with Dp < 0.01 µm ), "Aitken mode" (0.01 < Dp < 0.1 µm)), "accumulation mode" (0.1 < Dp < 1 µm), and "coarse mode" (Dp > 1 µm)

Solution Droplets

Note that the previous lecture discussion is valid for a pure water drop

• if a droplet is comprised of a solution - it can be in equilibrium with the environment at a much lower RH -->

• this explains the formation of haze• This process of condensation will

grow drops , but not to precipitation sizes (~ 2 mm)

Q: So, if a droplet grows to some size by condensation, how can it continue to grow to precipitation size???

(to be answers in next lecture)

QUESTION FOR THOUGHT:

• Haze particles can form when the relative humidity is less than 100%. Are these haze particles pure water droplets or solution droplets? Why?

numerator represents ___________denominater represents____________

Curvature EffectSolute Effect

Fig. 1 (Andreae and Rosenfeld, 2008)

Köhler Peak

Must know!

• Fig. 1. Köhler curves showing the equilibrium water vapor

supersaturation at 293 K for droplets of pure water (dotted curve) and for droplets containing various masses of dissolved (NH4)2SO4 (solid curves) vs. diameter of the droplet (Seinfeld and Pandis, 1998). The water vapor supersaturation, S (%)=(p / p0 − 1)100, where p is the partial pressure of the water vapor and p0 is the saturated vapor pressure over a plane surface of water at this temperature. In the indicated example, an ambient water vapor S of 0.15% (dashed line) exceeds the critical value for all ammonium sulfate aerosols with dry diameter ≥ 0.1 μm. These aerosols will therefore activate and grow into cloud droplets, whereas smaller aerosols remain as unactivated haze particles. Droplets below their corresponding equilibrium curve will shrink by evaporation whereas those above will grow by condensation (the indicated droplets correspond, for example, to a dry diameter of 0.05 µm).

http://www.sciencedirect.com/science/article/pii/S0012825208000317

Right of the peak:

• CCN is activated right of Kohler peak

• Continue to grow as long as water vapor is available

• Become cloud droplet

FYI

FYI

How is amount of solution change water droplet formation? Give example.

How different solution change water droplet formation?Give example.

Köhler Curve

5,6, 2,3,4 3,6

Droplet activated

• Find it in text book P214

CCN activation

• the CCN size distribution is dominated by the growing nucleation mode (Aitken Mode) rather than by the accumulation mode

• The activation of CCN determines the initial number of cloud droplets

Water Droplet Growth - Condensation

Growth slows down with increasing droplet size:

PHYS 622 - Clouds, spring ‘04, lect.4, Platnick

R&Y, p. 111

large droplets : dr

dt~

G senv r

Since large droplets grow slower, there is a narrowing of the size distribution with time.

FYI

Assumes supersaturation=0.05%, p=900 hPa, T=273K

rdry= 0.1 0.22 0.48 m

Class Participation

Diffusional growth summary:

• Accounted for vapor and thermal fluxes to/away from droplet.

• Growth slows down as droplets get larger, size distribution narrows.

• Initial nucleated droplet size distribution depends on CCN spectrum & ds/dt seen by air parcel.

• Inefficient mechanism for generating large precipitation sized cloud drops (requires hours). Condensation does not account for precipitation (collision/coalescence is the needed for “warm” clouds - to be discussed).

Water Droplet Growth - Condensation

Sinks of Aerosols

• Aerosols are removed from the atmosphere by dry and wet processes.

• For particles below Dp < 0.1 µm the dominant dry removal mechanism involves diffusion to the surface, a process which becomes less efficient as particle size increases.

• Coarse particles (Dp > 1 µm) settle gravitationally, a process which becomes less efficient as particle size decreases.

• In the range 0.1 < Dp < 1 µm, dry removal is very slow, these particles are removed mainly by growth to cloud drops during cloud formation and subsequent removal from the atmosphere in precipitation.

• The following Slides will be discussed in future lecture

Aerosol forcing has big uncertainty in current climate change research

Example of Effects of Aerosols

• They scatter visible sunlight:

– The sky looks dirty or hazy

– Creates brilliant orange and red skies at sunrise and sunset

– Creates a large aureole around the sun or full moon

Some act as nuclei formation of cloud drops or ice crystals

Photo taken by Carlye Calvin

© University Corporation for Atmospheric Research

Photo shows a sunset made possible by the aerosols in the air.

Photo taken by Carlye Calvin with copyright at

the University Corporation for Atmospheric Research.

Aerosol Radiative Forcing1) Direct forcing

Scattering and absorption of solar and terrestrial radiation by aerosols

2) Indirect forcing

Effects of aerosols on the optical properties and lifetime of clouds

surfaceCloud dropRain dropIce crystalIce precipitation

0oC

• An important type of primary particle is so-called "soot", emitted from combustion of carbonaceous fuels. It consists of black carbon containing material which has not been fully oxidized in the combustion process,

• Soot is formed in the combustion of carbonaceous fuels as particles with a diameter 5 to 20 nm

Aerosol Distributions over Land and Ocean have evident differences

July 2005

Satellite observations

January 2006

Seasonal variation of aerosol optical thickness (AOD) is evident

January 2006

Key Understanding Related to Aerosol Effects

(IPCC 2007 modified from Haywood and Boucher, 2000)

Much is still to be learned about how aerosols affect climate.

The aerosols that act as nuclei for formation of cloud drops are called cloud condensation nuclei (CCN).

An increase in the number of condensation nuclei increases the number of cloud droplets per unit volume, and decreases the droplet size. This increases the amount of sunlight reflected by the cloud. Similarly, fewer condensation nuclei mean fewer and larger cloud droplet per unit volume. Thus cumulus clouds over land (where there are a lot of condensation nuclei) have more small droplets than clouds forming over the open ocean, where there are usually fewer nuclei.

• According to traditional Köhler theory, a critical supersaturation (RH > 100%) exists above which water vapour condensed in an uncontrolled way and cloud droplets form. This critical supersaturation depends on particle size and chemical composition. Once a drop of sufficient volume is formed, gaseous species like SO2 can dissolve and be oxidized in the aqueous phase.

• On average, nine out of ten clouds evaporate rather than precipitate. When the droplets evaporate, larger particles reform as a result of the additional oxidized material, e.g. sulfate (Birmili, et al., 1999; Yuskiewicz, et al., 1999).

Importance of particle size

The dynamics of particles (biological and nonbiological)in the air is governed mainly byparticle physical characteristics of which size isone of the most important

Particle size matters!

Without aerosols, without clouds

Zonal Mean Cloud Effective Radius(M. D. King, S. Platnick et al. – NASA GSFC)

July 2006 (Collection 5)

Aqua

Why are cloud droplet sizes different over land and ocean?

AEROSOL!

Class Discussion

• http://www.nasa.gov/vision/earth/environment/ny_air_prt.htm

• Answer questions:

What is the role of urban aerosol on clouds droplet size and rainfall?

Total solar radiation decreased by aerosol= 20Wm-2

(Jin, Shepherd, and King, 2005, JGR)

Aerosol decreases surface insolation

Based on M-D. Chou’s radiative transfer model

surface

Key Understanding Related to Aerosol Effects

(IPCC 2007 modified from Haywood and Boucher, 2000)

Much is still to be learned about how aerosols affect climate.

Aerosol effects on Cloud Lifetime Effect

• Satellite studies show aerosol loading is positively correlated with low cloud fraction

• Interpreted as a result of longer cloud lifetimes caused by increasing aerosol number and reduced drizzle rates

Aerosol Number

Clo

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Sekiguchi et al. 2003 Rosenfeld et al. 2006

Class participation

• Open notes quiz

1. What is the typical size of aerosol?

2. What are the sources for aerosol?

3. What is the Aerosol direct effect?

4. What is aerosol indirect effect?